Remove dispvtx0 to avoid HLT truncation errors [ATR-29402]

HLT reprocessing tasks with the Dev menu have suffered from a large rate of truncation errors due to very large HLT_TrigDV_VSITrkPair collections. As there is no active development ongoing, the chain HLT_dispvtx0_loose_L1XE50 generating it can be removed. We had earlier commented out the related HLT_fsvsi0_L1XE50 chain as well, but for cleanliness let's just cut them all out. They can be reinstated as and when somebody wishes to follow up their development.